This invention relates to a method of treating wastewater sludge designed to decontaminate the sludge so that it can be safely applied as fertilizer to agricultural lands.
Romans used lime to disinfect and deodorize human waste. The use has continued throughout the development of civilization. However, prior to this invention, the use of lime for wastewater, sludge treatment has been severely limited by governmental regulations including the United States Environmental Protection Agency (EPA).
The EPA has promulgated rules governing the type of processes that can be used to treat wastewater sludge.
Under 40 CFR 257, a Process to Further Reduce Pathogens (PFRP) (See p. 5,6) must be used where sewage sludge or septic tank pumpings are to be applied to a land surface or are incorporated into the soil, and crops for direct human consumption are to be grown on such land within eighteen (18) months subsequent to application or incorporation.
A Process to Significantly Reduce Pathogens (PSRP) (See p. 5) must be used where sewage sludge or septic tank pumpings are to be applied to a land surface or incorporated into the soil and the public will have access to such land within twelve (12) months subsequent to application or incorporation, or grazing animals, whose products are consumed by humans, will have access to such land within one (1) month subsequent to application or incorporation.
Appendix II of 40 CFR 257 classifies the following as PSRP and PFRP processes:
A. Processes to Significantly Reduce Pathogens
Aerobic digestion: The process is conducted by agitating sludge with air or oxygen to maintain aerobic conditions at residence times ranging from 60 days at 15xc2x0 C. to 40 days at 20xc2x0 C., with a volatile solids reduction of at least 38 percent.
Air Drying: Liquid sludge is allowed to drain and/or dry on under-drained sand beds, or paved or unpaved basins in which the sludge is at a depth of nine inches. A minimum of three months is needed, two months of which temperatures average on a daily basis above 0xc2x0 C.
Anaerobic digestion: The process is conducted in the absence of air at residence times ranging from 60 days at 20xc2x0 C. to 15 days at 35xc2x0 to 55xc2x0 C., with a volatile solids reduction of at least 38 percent.
Composting: Using the within-vessel, static aerated pile or windrow composting methods, the solid waste is maintained at minimum operating conditions of 40xc2x0 C. for 55 days. For four hours during this period the temperature exceeds 55xc2x0 C.
Lime Stabilization: Sufficient lime is added to produce a pH of 12 after 2 hours of contact.
Other methods: Other methods or operating conditions may be acceptable if pathogens and vector attraction of the waste (volatile solids) are reduced to an extent equivalent to the reduction achieved by any of the above methods.
B. Processes to Further Reduce Pathogens
Composting: Using the within-vessel composting method, the solid waste is maintained at operating conditions of 55xc2x0 C. or greater for three days. Using the static aerated pile composing method, the solid waste is maintained at operating conditions of 55xc2x0 C. or greater for three days. Using the windrow composting method, the solid waste attains a temperature of 5xc2x0 C. or greater for at least 15 days during the composting period. Also, during the high temperature period, there will be a minimum of five turnings of the windrow.
Heat drying: Dewatered sludge cake is dried by direct or indirect contact with hot gases, and moisture content is reduced to 10 percent or lower. Sludge particles reach temperatures well in excess of 80xc2x0 C., or the web bulb temperature of the gas stream in contact with the sludge at the point where it leaves the dryer is in excess of 80xc2x0 C.
Heat treatment: Liquid sludge is heated to temperatures or 180xc2x0 C. for 30 minutes.
Thermophilic Aerobic Digestion: Liquid sludge is agitated with air or oxygen to maintain aerobic conditions at residence times of 10 days at 55-60xc2x0 C., with a volatile solids reduction of at least 38 percent.
Other methods: Other methods of operating conditions may be acceptable if pathogens and vector attraction of the waste (volatile solids) are reduced to an extent equivalent to the reduction achieved by any of the above methods.
Any of the processes listed below, if added to the processes described in Section A above, further reduce pathogens. Because the processes listed below, on their own, do not reduce the attraction of disease vectors, they are only add-on in nature.
Beta ray irradiation: Sludge is irradiated with beta rays from an accelerator at dosages or at least 1.0 megarad at room temperature (ca. 20xc2x0 C.).
Gamma ray irradiation: Sludge is irradiated with gamma rays from certain isotopes, such as 60Cobalt and 137Cesium, at dosages of at least 1.0 megarad at room temperature (ca. 20xc2x0 C.).
Pasteurization: Sludge is maintained for at least 30 minutes at a minimum temperature of 70xc2x0 C.
Other methods: Other methods of operating conditions may be acceptable if pathogens are reduced to an extent equivalent to the reduction achieved by any of the above add-on method.
Prior to this invention, many concerns have been raised about the long term disinfection and stabilization capability of lime treatment. Parrel et al, in xe2x80x9cLime Stabilization of Primary Sludgesxe2x80x9d, Journal of Water Pollution Control Fed 46, 113 Jan. 1974 published by USEPA, states: xe2x80x9cLime stabilization does not make the sludges chemically stable. The pH eventually falls and surviving bacteria may return if conditions are favorable. . .higher organisms such as Ascaris survive short term exposure to pH of 11.5 and possibly long term exposure.xe2x80x9d
In January 1979, the EPA published a Wastewater Sludge Manual (EPA 625/1-79-001) titled xe2x80x9cProcess Design Manual for Sludge Treatment and Disposalxe2x80x9d which states:
xe2x80x9cLime stabilizations a very simple process. Its principal advantages over other stabilization processes are low cost and simplicity of operation. . .lime addition does not make sludges chemically stable; if pH drops below 11.0, biological decomposition will resume producing noxious odors. Second, the quantity of sludge for disposal is not reduced as it is by biological stabilization methods. On the contrary, the mass of dry sludge is increased by the lime added and by the chemical precipitates that derive from the addition. Thus because of the increased volume, the costs of transport and ultimate disposal are often greater for lime stabilized sludges than for sludge stabilized by other method. . .quantitative observation under a microscope has shown substantial survival of higher organisms, such as hook worms, amoebic systs and Ascaris ova after contact time of 24 hours at high pH.xe2x80x9d
Reimers, Englande et al (EPA 600/2-81-166) reported that:
xe2x80x9cApplication of like to primary aerobic digested and anaerobic digested sludge was found to be effective with greater than 80% reduction of Ascaris viability in 5 days following aerobic digestion at a lime dosage of about 1000 mg/gram of sludge solids (one part lime to one part sludge solids). . . In the case of the 35xc2x0 C. aerobically-digested sludge, there was no apparent effect of lime on the viability of Ascaris eggs at dosages up to 3000 mg of lime per gram of dry sludge solids under anaerobic conditions, in the period of 20 days. However, under aerobic conditions, a 98% reduction of viable Ascaris eggs was observed within one hour at dosages greater than 1000 mg of lime per gram of dry sludge solid, but only 77% reduction of the viable eggs was observed at a dosage of 100 mg lime per gram of dry sludge solids after 20 days. The explanation of these differentials is not apparent.xe2x80x9d
In July 1984, the Sandia National Laboratories published a report titled xe2x80x9cPathogens in Sludge Occurrence, Inactivation and Potential for Regrowthxe2x80x9d which states:
xe2x80x9cTo summarize the effects of lime on sludge pathogens viruses are destroyed by high pH values, although it has not been shown that viruses within sludge itself are inactivated; parasite ova are resistant to high pH, and most will probably survive lime treatment; bacteria are rapidly inactivated at pH 12 but, because of pH decreases at levels suitable for bacteria growth, their numbers increase with time.xe2x80x9d
In October 1984, the EPA published a report (EPA 625/10-84-003) titled xe2x80x9cUse and Disposal of Municipal Wastewaster Sludgexe2x80x9d which was the basis for future regulations. Section 3 of the report states:
xe2x80x9cIf crops for direct human consumption are grown within 18 months of sludge application, sludge must be treated with a PFRP. These processes destroy pathogenic bacteria, viruses and protozoa as well as parasites in most cases by exposing the sludge to elevated temperatures over a period of time.xe2x80x9d
On November 6, 1985 the EPA issued a memorandum regarding application of 40 CFR 257 regulations to pathogen reduction preceding land application of sewage sludge or septic tank pumpings. One of the purposes of issuing the memorandum was to outline procedures to enable enforcement agents to determine whether processes other than those listed in the regulation (40 CFR 257) qualify as a PFRP process. To qualify a process as a PSRP, one must demonstrate that the process reduces animal viruses by one log and pathogenic bacterial densities by at least two logs and must reduce the vector attractiveness such that vectors, like flies or rats, are not attracted to the sludge. To qualify a new process as PFRP, one must demonstrate reduction of pathogenic bacteria, animal viruses, and parasites xe2x80x9cbelow detectable limitsxe2x80x9d of one (1) plaque forming unit (PFU) per 100 ml of sludge for animal viruses; three (3) colony forming units (CFU) per 100 ml of sludge for pathogenic bacteria (Salmonella sp.); and one (1) viable egg per 100 ml of sludge for parasites (Ascaris sp.). Vector attractiveness must also be reduced for PFRP.
If only PSRP disinfective is utilized, land application for fertilization purposes is controlled by EPA restrictions (it cannot be used on root crop: xe2x80x9c40 CFR 257xe2x80x9d). If the process achieves PFRP criteria these restrictions are eliminated (xe2x80x9c40 CFR 257xe2x80x9d).
In my U.S. Pat. No. 4,554,002, it was shown that kiln dust could be used to reduce pathogens and dry wastewater sludge prior to land application.
Roediger, U.S. Pat. No. 4,270,279, describes a method of drying and sterilizing sewage sludge wherein sheet-like sewage sludge is broken up into ball-like sludge particles and dusting the outer surface only with quicklime. This technology utilizes exothermic heat generated from the reaction of adding H2O to quick lime to sterilize the sludge. This heat sterilization is typical to the traditional aforementioned PFRP processes. To this date, the EPA has not approved a petition for approval of this technology as a PSRP process. Moreover, there are problems with this method. If this method actually sterilizes the sludge, it would kill all life forms contained in the sludge, whether they were pathogenic or beneficial non-pathogenic microorganisms. In contrast, the present invention decontaminates sludge, killing pathogens to a level below PFRP standards but does not eliminate all nonpathogenic microorganisms from the sludge.
None of the above references suggest that lime or kiln dust, in combination with a natural drying process, could be used to produce the pathogenic reducton in wastewater sludge equivalent to PFRP processes, and thus provide an inexpensive method of treating wastewater sludge such that it can be applied directly to land as a fertilizer to grow crops for direct human consumption.
In accordance with the invention, lime, cement kiln dust or lime kiln dust or mixtures thereof and/or other alkaline materials are mixed with wastewater sludge in sufficient quantity to raise the pH to 12 and above for at least two hours and the resulting mixture is actively dried by an aeration process. The process produces a product wherein the pathogen viability has been reduced to a level that meets or exceeds USEPA criteria for PFRP processes without eliminating all of the beneficial non-pathogenic microorganisms.